This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. Such photoimageable compositions are useful for various purposes in the manufacture of printed circuit boards. This invention is particularly applicable to primary imaging photoresists which are used in the creation of the printed circuitry itself.
A variety of such negative-acting photoimageable compositions have been described. The essential ingredients include A) a film-forming binder polymer; B) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) usually has sufficient acid functionality, generally carboxylic acid functionality, so that it is soluble (until exposed to actinic radiation) in dilute alkaline aqueous solutions and thus renders the photoimageable composition developable in the same. The binder polymer A) in the polymerized state (after exposure to actinic radiation) is resistant to the developing, etching and plating solutions, but retains sufficient acid functionality which renders the photoimageable composition releasable or strippable in strong alkaline aqueous solutions. Furthermore, the standard binder polymer A) usually comprises a single monodisperse polymer identifiable by a single narrow peak on a molecular weight distribution graph.
One problem with the use of such photoimageable compositions as photoresists is that they are difficult to strip from electrolytically plated circuit boards using conventional alkaline aqueous stripping solutions, e.g., 3% NaOH solution. This problem arises from the recent demand on circuit board manufacturers to reduce the size of printed circuit boards, while also increasing their functional capabilities. Consequently, the circuit lines and spaces on the circuit boards have continued to shrink, as more circuitry needs to be fit onto smaller surfaces. At the same time, metal plating heights have also increased above the thickness of the photoresist. This causes the metal to hang over the photoresist, resulting in a very narrow space containing the photoresist being virtually encapsulated by the overplating. The photoresist then becomes trapped by the plated overhang, making it difficult to attack and strip by conventional methods. If the resist is not stripped clean, ragged copper circuit lines will result after etching which are unusable, since they can cause short circuiting of the board.
Some circuit board manufacturers have tried thicker photoresists to accommodate the increasing plating heights. But this approach is more expensive and limits resolution of the circuit lines. The majority use organic-based (amine- or organic solvent-containing) alkaline stripping solutions, which produce a smaller stripped particle to facilitate stripping. While these organic strippers, e.g., solutions containing trimethylamine or tetramethylammonium hydroxide, remove the resist better, they are expensive (relative to NaOH or KOH) and have more waste treatment and environmental concerns associated with them. Moreover, due to the recent emphasis on reducing solvent emissions in the workplace, solvent-strippable photoresists are much less desirable than those that are aqueous-strippable.
It, therefore, would be desirable to provide photoimageable compositions useful as photoresists which do not suffer from the foregoing disadvantages.
This invention provides photoimageable compositions useful as photoresists that exhibit improved stripping properties, and that can be quickly stripped from overplated circuit traces in environmentally favorable, physiologically safe, fully aqueous, alkaline stripping solutions, without compromising the quality of the patterned circuit lines.